There isn't a way to do this. A few months back, a moderator showed up and tempted us with the idea of putting in custom exercise, like custom food, then it never came to pass and the moderators have all dropped off a cliff, apparently. Your best option on this site is to find something that gives you a similar burn amount for the amount of time.
I hear your frustration...it's probably the biggest drawback of this site, IMO.
"If you drop an egg, you don't say, 'Oh, shoot' and drop the other 11, do you?"
-Source unknown, but obviously brilliant
Reached goal 4/16/2010...but kind of afraid to look these days